1. Field of the Invention
This invention relates in general to magnetic transducers for reading information signals from a magnetic medium and, in particular, to an improved magnetoresistive read transducer and a method for making the improved transducer.
2. Description of the Prior Art
The prior art discloses a magnetic transducer referred to as a magnetoresistive (MR) sensor or head which has been shown to be capable of reading data from a magnetic surface at great linear densities. An MR sensor detects magnetic field signals through the resistance changes of a read element made from a magnetoresistive material as a function of the amount and direction of magnetic flux being sensed by the element.
The prior art also teaches that in order for a MR sensor to operate optimally, two bias fields should be provided. In order to bias the material so that its response to a flux field is linear, a transverse bias field is generally provided. This bias field is normal to the plane of the magnetic media and parallel to the surface of the planar MR sensor.
The other bias field which is usually employed with MR sensors is referred to in the art as the longitudinal bias field, which extends parallel to the surface of the magnetic media and parallel to the lengthwise direction of the MR sensor. The function of the longitudinal bias field is to suppress Barkhausen noise, which originates from multi-domain activities in the MR sensor.
A MR sensor for reading information signals from a magnetic recording medium is described in U.S. Pat. No. 4,103,315 to Hempstead, et al., which is assigned to the same assignee as this application. The '315 patent describes a MR read sensor which utilizes antiferromagnetic-ferromagnetic exchange coupling to produce a uniform longitudinal bias in the MR layer of the sensor.
The materials suggested by the '315 patent are nickel-iron (NiFe) as the ferromagnetic MR layer and a manganese (Mn) alloy as the antiferromagnetic layer. Of the possible Mn alloys, iron-manganese (FeMn) appears to exhibit the greatest ability to exchange couple with the NiFe layer, and the FeMn is deposited directly on the NiFe to obtain the exchange bias effect.
The strength of the exchange bias field for exchange coupled films of sputter deposited NiFe and FeMn films has been studied by Tsang, et al. in "Exchange Induced Unidirectional Anisotropy at FeMn-Ni.sub.80 Fe.sub.20 Interfaces", J. Appl. Phys. Vol. 52 (3), March, 1981, pp. 2471-2473.
The use of annealing between Mn and NiFe to obtain an antiferromagnetic alloy of high ordering temperature (250.degree. C.) was described by Massenet, et al, in "Magnetic Properties of Multilayer Films of FeNi--Mn--FeNiCo and of FeNi--Mn", IEEE Tran. on Mag. MAG-1, March, 1965, p. 63. In the publication "Spin-Wave Resonance in Epitaxial Fe--Ni Films and in Coupled Double Layers of Epitaxial Fe--Ni and Fe--Ni--Mn", Waksmann et al., Journal of Applied Physics V39, No. 2, February, 1968, p. 1389, the authors directly evaporated an NiFeMn ternary (around the ordered Ni.sub.50 Mn.sub.50 region) onto NiFe, and obtained exchange bias which was fairly low (7 Oe. at 400A NiFe) but an ordering temperature which was high (250.degree. C).
The teachings of the prior art have enabled the design of MR sensors which meet prior art requirements. However, the drive toward increased recording density has led to the requirement for narrower recording tracks and increased linear recording density along the tracks. The small MR sensors which are necessary to meet these requirements cannot be made with the use of the prior art techniques due to the low magnitude of the exchange bias field that can be obtained, the pronounced temperature dependence of the magnitude of the exchange bias field, and the low ordering temperature at which the exchange bias field goes to zero.